The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube in which the effective region of a shadow mask is enlarged and the magnetic shielding characteristic and the howling prevention characteristic for assembling the shadow mask are improved.
A color cathode ray tube generally comprises a vacuum envelope (glass bulb) provided with a panel which is arranged to close the front side and on which a fluorescent screen is formed, a slender neck portion which is arranged at the rear side and in which an electron gun is mounted, and a funnel connecting the panel and the neck portion. A shadow mask assembly is fixed in front of the panel inside the envelope; a magnetic shield is set in the tube in the vicinity of the joint between the panel and the funnel; and a deflection yoke is set to surround the tube at the joint between the funnel and the neck portion. Moreover, the shadow mask assembly includes a shadow mask having a surface curved toward the panel, and in which a plurality of electron beam passing holes are formed, and a skirt extending away from the margin of the curved surface, and a mask frame having a flange connected to a substantially rectangular side wall and to the magnetic shield, which is constituted by fitting and securing the skirt to the side wall. Moreover, the shadow mask assembly is secured in the panel so that the curved surface of the shadow mask faces the fluorescent screen formed on the front panel.
In the case of a color cathode ray tube having the above structure, three electron beams are emitted from three electron guns included in the neck portion of the envelope and the three beams are properly deflected by a magnetic field generated by the deflection yoke. Then, the three electron beams pass through the electron beam passing holes formed on the curved surface of the shadow mask and, thereafter, each of them is projected on a phosphor region having a corresponding color on the fluorescent screen. Thus, a multi-color image is displayed on the front panel of the color cathode ray tube.
FIG. 15 is a top view showing the structure of a shadow mask assembly of the type used for an already-known color cathode ray tube.
In FIGS. 15 and 16A and 16B, symbol 40 generally denotes a shadow mask assembly, 41 generally denotes a shadow mask, 42 denotes a curved surface, 43 denotes a skirt, 44 generally denotes a mask frame, 45 denotes a side wall of the mask frame, 46 denotes a spring for supporting the shadow mask assembly in the panel of the tube envelope, 47 denotes a boss formed on the side wall of the mask frame and directed toward the inside of the frame, and 48 denotes a flange of the mask frame.
Moreover, the shadow mask 41 includes the curved surface 42 having a plurality of electron beam passing holes (not illustrated) and the skirt 43 extending from the margin of the curved surface 42. The mask frame 44 has the side wall 45 and the flange 48 which are connected to each other or press-molded into a rectangular shape. The spring 46 for supporting the mask frame 44 in the panel is attached to the outside of the side wall 45. Moreover, the mask frame has a plurality of bosses 47 for reinforcing the mechanical strength of the side wall 45 and for compensating for the clearance of the joint face between the side wall 45 and the skirt 43. The skirt 43 of the shadow mask 41 is fitted to the side wall 45 of the mask frame 44, and the skirt 43 and the side wall 45 are spot-welded to each other at several locations.
FIG. 16A is a sectional view of a major side of the shadow mask shown in FIG. 15, and FIG. 16B is a sectional view of a corner of the shadow mask assembly shown in FIG. 15. When the shadow mask 41 is press molded, a warp occurs due to springback because of the press-molding. The warp value SB of the skirt 43 usually tends to decrease at the corners with a relatively large degree of contraction due to press-molding and to increase at the central portion with a relatively small degree of contraction. The bottom margin of the skirt of the shadow mask 41 after being press-molded has a shape in which the middle of the side is slightly curved outward compared to the portion near a corner due to the warping. When fitting the skirt 43 of the shadow mask 41 into the side wall 45 of the mask frame 44, there is a problem that the skirt 43 contacts the side wall 45 of the frame 44 at the middle of a side thereof, while a clearance 49 is produced between the skirt 43 of the shadow mask 41 and the mask frame 44 at a corner.
Moreover, the margin of the curved surface 42 is molded so that the middle of a side is slightly curved outward in accordance with the shape of the margin of the glass panel. That is, to fit the skirt 43 to the side wall 45 of the mask frame, the dimensions of various portions of the shadow mask 41 are set so that the middle of a side of the skirt 43 just meets the side wall 45.
Moreover, when fitting the skirt 43 of the shadow mask 41, which has been press-molded at the above dimensions, into the side wall 45 of the mask frame 44, the clearance between the skirt 43 and the side wall 45 decreases at the middle of a side of the skirt 43 and increases at the corners of the skirt 43. Therefore, as shown in FIG. 15, a boss 47 is formed at each corner of the side wall 45 so as to compensate for the increasing clearance. A boss 47 is also properly formed at the middle of a side of the side wall 45 in order to reinforce the mechanical strength of the side wall 45.
Thus, the well-known shadow mask assembly 40 is molded so that the middle of a side of the margin of the curved surface 42 is slightly curved toward the outside when press-molding the shadow mask 41. Therefore, the clearance between the skirt 43 and the side wall 45 increases at the corners of the skirt 43. Moreover, problems occur in that it is necessary to form a boss 47 at each corner of the side wall 45, so that the effective region of the curved surface 42 relative to the size of the mask frame 44 is narrowed by a value equivalent to the increase in the clearance.
Furthermore, in the case of the well-known shadow mask assembly 40, the contact area between the skirt 43 and the side wall 45 is restricted to the arranged portion of the boss 47 at each corner. Therefore, the contact area between the skirt 43 and the side wall 45 is substantially small. In addition, because the contact area is small at the corners, there is a problem that an effective magnetic shielding characteristic cannot be obtained.
The present invention is made to solve the above problems and its object is to provide a color cathode ray tube comprising a shadow mask assembly which is capable of enlarging the effective region of the curved surface thereof to the size of the mask frame of the shadow mask assembly, thereby obtaining a superior magnetic shielding characteristic.
Though U.S. Pat. No. 4,308,485 discloses an example of the prior art, it does not suggest the present invention. Moreover, Japanese Utility Model Application No. 40942/1985 discloses a prior art arrangement, in which springback when forming a shadow mask is considered. However, neither the shape of the mask frame for holding the shadow mask nor the positional relation between the shadow mask and the mask frame are disclosed at all.
To achieve the above object, the present invention mainly comprises the following features.
(1) The side of the margin of the curved surface of a substantially rectangular shadow mask is set so that it is curved inward. That is, the side is set to a curvature which is convex to (toward) the tube axis of the cathode ray tube. Moreover, the substantially rectangular side wall of a mask frame is set so as to be linear or so as to.have a curvature which is concave toward the tube axis of the cathode ray tube. Thereby, it is possible to obtain a sufficient clearance between the mask frame and the shadow mask for major and minor axes of the rectangle. Thus, it is possible to retain the mass productivity of the shadow mask even if the clearance is small on the diagonal axis. Moreover, it is possible to improve the magnetic shielding effect (effect of shielding against magnetism from outside the cathode ray tube) on the diagonal of a screen where the influence of geomagnetism is maximized. Furthermore, it is possible to increase the diagonal effective diameter of the screen by a value equivalent to the decrease of the clearance due to the diagonal axis.
(2) The side of the margin of the curved surface of a substantially rectangular shadow mask is set so as to be linear or to have a radius of curvature in which the side is curved outward and the radius of curvature of the side of the margin of the shadow mask is made larger than that of the inside of the side wall of a mask frame corresponding to the side of the margin of the shadow mask. Thereby, because a sufficient clearance is obtained between the mask frame and the shadow mask for major and minor axes of the above rectangle, it is possible to retain the mass productivity of a shadow mask assembly by decreasing the clearance on the diagonal axis of the rectangle.
(3) Because a boss is formed at a portion including the root on which the flange of the side wall of the mask frame is set and the skirt of the shadow mask is secured on the boss, it is possible to maintain the mechanical strength of the mask frame, increase the effective diameter of the shadow mask, and, moreover, to improve the magnetic coupling between the shadow mask and the mask frame.
(4) The clearance between the mask frame and the margin of the shadow mask is decreased at the corners and a boss is provided from the proximal edge of the side wall with the flange at the side of the mask frame up to the distal edge of the side wall to decrease the clearance at this portion. Thus, because the small region of the clearance is limited, it is possible to retain the mass productivity of a shadow mask assembly and obtain the advantages indicated in the above Items (1) to (3).
Therefore, according to the above features, the curved surface of a shadow mask is substantially enlarged at the corners and the effective region of the curved surface is expanded. Moreover, because the contact area between the skirt and the side wall is expanded at the corners, the magnetic shielding characteristic is improved at the corners of the shadow mask assembly.